This invention relates to the processing of rotating titanium alloy articles to improve the microstructure of such articles.
A wide range of titanium alloys are used in airframe and gas turbine engines for aerospace applications. Considerable research has been directed toward increasing strength and fatigue properties of critical titanium alloy parts, such as airframe and gas turbine compressor components.
Due to the nature of titanium transformation and alloying stabilization behavior, titanium grades can be grouped into three major classes, depending on the phase or phases present in their microstructures. These are alpha/near-alpha, alpha+beta, and near-beta/beta types.
Most titanium alloys currently used for high performance aerospace applications are alpha+beta (e.g., Ti-6Al-4V, a typical airframe alloy) and near-alpha (e.g., Ti-6Al-2Sn-4Zr-2Mo, a typical gas turbine engine compressor alloy) alloys. Commercial emphasis for the manufacture of these alloys has been largely placed on alpha+beta processing to assure adequate strength and ductility. Alpha+beta alloys are the most commonly used titanium alloys and are designed for intermediate strength and high fracture resistance in both airframe and engine applications. Near-alpha alloys possess excellent high temperature properties and are generally designed for high creep properties at high temperatures. Because of lack of toughness in the solution treated and aged condition and relatively poor hardenability, alpha+beta alloys have commonly been used in the annealed condition. As a result, the strength capability of titanium alloys cannot be effectively utilized.
Forging of near-alpha or alpha+beta titanium alloys is one of the most common methods for producing high integrity components for fatigue-critical airframe and gas turbine engine applications. Currently, forging of these classes of alloys is done at temperatures below the beta transus temperature of the alloys because the microstructures developed have a good combination of tensile and fatigue properties. On the other hand, forging near or above the beta transus temperature provides certain advantages in terms of reduced press load and much better shape definition, since the alloy plastic flow resistance is greatly reduced. Unfortunately, the microstructure developed as a result of such forging is a lenticular beta microstructure which is inferior in terms of fatigue performance.
Fatigue failures account for the majority of aircraft in-service component failure. Fatigue failure is divided into crack initiation and crack propagation stages.
In recent years, more and more rotating axisymmetric components are made of titanium alloys. This is due to the relatively low density of titanium which lowers the centrifugal and hoop stresses and subsequently reduces the bearing loads in rotating jet engines. At the same time, high frequency rotation exerts high levels of mechanical vibration, the result of system imbalance or interruption in air or gas flow. This leads to fatigue failure so common in these components.
It is known from titanium metallurgy that fine equiaxed structure, such as that developed during recrystallization treatment, is highly fatigue crack initiation resistant, while beta processed lenticular alpha structure, such as developed during beta extrusion, beta forging or beta anneal, is highly fatigue crack propagation resistant, but inferior in fatigue crack initiation resistance.
By locating fine equiaxed structure in potential crack initiation sites and lenticular beta processed structure in the crack propagation sites, it is possible to obtain components with superior fatigue resistance. It should be noted that in most components, fatigue cracks initiate at or close to the surface and propagate into the bulk of the material. To date, it has been necessary to resort to processing methods such as shot-peening to achieve such partitioned microstructure. Shot-peening of beta processed titanium alloy components has not been successful due to inherent surface cracking, the result of shear band deformation at the surface during the shot peening impact.
What is desired is a method for producing two different microstructural zones in one component or article in a relatively simple manner.
Accordingly, it is an object of the present invention to provide an improved process for producing near-alpha and alpha+beta titanium alloy axisymmetric components with high fatigue resistance.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the invention.